One process of determining the capacity of a discharging battery is based on the ampere hour capacity of the battery; that is the number of hours that the battery can supply a given current, assuming a constant current, or supply a given ampere hour area if the current varies. Capacity evaluation is based on current because it is a reliable indication of the ability of the battery to power a load. The discharge of the battery is generally considered complete when the battery voltage drops to some voltage level which is a bare minimum requirement of the load network. Typical discharge characteristics of new batteries are shown in FIG. 1 in which discharge curves relating battery voltage and time are shown for various constant value discharge currents. It is readily apparent that the value of the discharge current significantly affects the time of discharge until a particular battery discharge voltage is reached. These discharge curves characteristically reflect the capacity of the battery when these measurements were made and would not necessarily reflect true battery capacity at a later stage in the battery's life.
Prior art methods of predicting the available reserve time remaining in a discharging battery were based on the assumption that all the battery cells retain their original discharge characteristics. The duration of a discharge to a specified end voltage was essentially predicted from power data-sheet information based on the experimental measured discharge characteristics of a new battery.
One widely used method of predicting the available reserve time of a discharging battery is based on the Peukert parameters, which are based on measured constants that depend on the end voltage of interest of a discharging battery and which are included in an exponential equation. Plotted logrithmically the discharge characteristic of a battery is a straight line with a separate straight line for each end voltage value. Implicit in this method and other prior art methods is the premise that the battery discharge characteristic is fixed and does not vary with the age of the battery. Furthermore the prior art methods assume that the discharge current is a constant value for the entire discharge and do not accommodate varying discharge currents. These discharge prediction methods accordingly permit only rough approximations of the available reserve time of a discharging battery.
In reality the discharge characteristics of the battery cells deviate from their original behavior with age and these initial characteristics are retained for only a few initial charge/discharge cycles. Discharge characteristics also typically deviate substantially from those theoretically predicted by the Peukert relation. Hence it has not been feasible to provide reliable predictions of remaining reserve time for a mature discharging battery.